1. Field of the Invention
The present invention relates generally to radio communications and more specifically to communication technologies for multiple access in difficult and hostile environments combined with dynamic environment changes.
2. Description of the Prior Art
The communication technology developed in the 1940""s during World War II included xe2x80x9cfrequency diversity communicationxe2x80x9d or xe2x80x9cstacked carrier communicationsxe2x80x9d to aid high frequency (HF) band traffic. J. Proakis refers to frequency diversity communication technology in, Digital Communications, McGraw-Hill, 1989, see, sections 7.4 to 7.7. Diversity techniques are said by Proakis to be based on the notion that errors occur in the reception of largely attenuated channels, e.g., channels in deep fade. Supplying the receiver with several duplicates of the original signal, but over channels that fade independent from one another, has the potential of securing continuous communication except during the unlikely event that all the duplicate channels fade out together. Such probability can be estimated.
Frequency diversity is one of many diversity methods. The same modulation is carried by several carrier channels separated by nominally the coherence bandwidth of each respective channel. In time diversity, the same information is transmitted over different time slots.
Multiple antennas can be used in a diversity scheme. Several receiving antennas can be used to receive the signals sent from a single transmitting antenna. For best effect, the receiving antennas are spaced enough apart to vary different multipath interference amongst the group. A separation of nominally ten wavelengths is generally needed to observe independent signal fading.
A signal having a bandwidth much greater than the coherence bandwidth of the channel can be used in a more sophisticated diversity scheme. Such a signal with a bandwidth W will resolve the multipath components and provide the receiver with several independently fading signal paths.
Other prior art diversity schemes have included angle-of-arrival or spatial diversity and polarization diversity.
When a bandwidth W much greater than the coherence bandwidth of each respective channel is available to a user, the channel can be subdivided into a number of frequency division multiplexed sub-channels having a mutual separation in center frequencies of at least the coherence bandwidth of each respective channel. The same signal can then be transmitted over the frequency-division multiplex sub-channels to establish frequency diversity operation. The same result can be achieved by using a wideband binary signal that covers the bandwidth W.
G. K. Kaleh describes such in an article, xe2x80x9cFrequency-Diversity Spread-Spectrum Communication System to Counter Band-limited Gaussian Interferencexe2x80x9d, IEEE Transactions on Communications, September 1994. Here a secure setup is outlined that can operate in deliberately hostile signal environments.
J. Proakis describes frequency diversity spread spectrum and multiple access concepts in chapter eight, xe2x80x9cSpread Spectrum Signals for Digital Communicationxe2x80x9d, supra. Diversity transmission combined with frequency-hopping spread spectrum is detailed for protection against multipath fading and partial-band jamming.
Retro-directivity was proposed and used as early as 1959 to adapt a multi-element antenna array to provide identical spatial gain patterns during transmission and reception operations. See, R. Monzingo, T. Miller, Introduction to Adaptive Arrays, Wiley Interscience Publications, 1980; L. Van Atta, xe2x80x9cElectromagnetic Reflection,xe2x80x9d U.S. Pat. No. 2,908,002, 1959; and B. Glance, P. Henry, xe2x80x9cHigh Capacity Mobile Radio System,xe2x80x9d U.S. Pat. No. 4,383,332, May 10, 1983, for a discussion of such techniques. TDD systems provide an effective means for implementing retrodirective antenna arrays, e.g., by minimizing the channel variation between the reception and transmission paths.
It is therefore an object of the present invention to provide a radio communication system for spreading data over widely separated frequency bands manifesting differences in channel distortion without physically spreading signals between intervening frequencies, as is required with direct-sequence spread spectrum.
It is another object of the present invention to provide a radio communication system for communication under strong narrow-band interference, e.g., conventional cellular signal waveforms, by turning off affected frequency channels at a receiver""s despreader.
It is an object of the present invention to provide a radio communication system with simple equalization of linear channel multipath distortion.
It is an object of the present invention to provide a radio communication system that is compatible with discrete multitone and orthogonal frequency-division multiplex-like channelization techniques. And that is compatible with time-packetized discrete multitone and orthogonal frequency-division multiplex-like modulation/demodulation techniques for frequency channelization and inverse channelization.
It is another object of the present invention to provide a radio communication system that is compatible with time-division duplex systems where the stacked-carrier spread spectrum modulation format is packetized, e.g., if the stacked-carrier spread spectrum signal is generated using discrete multiple tone and/or orthogonal frequency-division multiplex-like based frequency channelizers and inverse channelizers.
It is an object of the present invention to provide a radio communication system for frequency-division multiple-access like multiple access capability.
It is an object of the present invention to provide a radio communication system for code-division multiple-access like capability in a stacked carrier multiple access arrangement.
It is an object of the present invention to provide a radio communication system compatible with high-order digital modulations.
It is an object of the present invention to provide a radio communication system for bandwidth-on-demand flexible data rate connections.
It is an object of the present invention to provide a radio communication system for space-division multiple-access like multiple access, interference excision, and channel equalization capability in a code nulling application.
It is an object of the present invention to provide a radio communication system for use with adaptive antenna arrays by spatially extending a spreading code to spread data using independent complex gains on each spatial channel, or antenna beam, to control the channel-bandwidth array dispersion.
It is an object of the present invention to provide a radio communication system compatible with advanced array adaptation techniques, e.g., non-blind pilot-directed, blind data-directed, and other techniques that take advantage of underlying properties of the baseband data, channel structure, or stacked carrier spreading format.
It is an object of the present invention to provide a radio communication system compatible with retrodirective communication techniques.
It is an object of the present invention to provide a radio communication system for back-compatibility with conventional code-division multiple access, data activation systems.
Briefly, an embodiment of the present invention comprises a xe2x80x9cstacked-carrierxe2x80x9d spread spectrum communication system wherein the spreading is done in the frequency domain by multiplying a time-domain representation of a baseband signal by a set of superimposed, or stacked, complex sinusoid carrier waves. In practice, the spreading is done by simply energizing the bins of a large fast Fourier transform (FFT). This provides a considerable savings. in computational complexity for moderate output FFT sizes. A Kaiser-Bessel window, e.g., with xcex2=9, can be used to xe2x80x9cfill outxe2x80x9d the space between the tones without subjecting those tones to unacceptable interference from adjacent tones, e.g., inter-tone interference. In particular, a high value of xcex2 provides acceptable interference between adjacent tones and extremely low interference between farther out tones. This basic technology is then combined with time-division duplex, code-division multiple access, space-division multiple access, frequency-division multiple access, adaptive antenna array and interference cancellation techniques.
An advantage of the present invention is that a radio communication method is provided that spreads data over widely separated frequency bands for spectral diversity. This provides an efficient way to take advantage of frequency diversity, especially in applications where the bands are very widely separated.
An advantage of the present invention is that a radio communication method is provided that communicates even under strong narrow-band interference. So a stacked-carrier spread spectrum (SCSS) link can be maintained in the presence of strong narrow-band frequency-division multiple access (FDMA) and time-division multiple access (TDMA) cellular radio signals, as in cellular overlay applications. It also allows such a link to be maintained in the presence of spurious interference due to harmonics from out-of-band signals.
An advantage of the present invention is that a radio communication method is provided that permits simple equalization of linear channel distortion, .and allows stationary, or quasi-stationary, linear channel distortion to be approximated as a multiplicative effect on the transmit. spreading code. It further allows the channel equalization operation to be subsumed into the despreading or spreading operation without additional filtering operations, apart from removal of intrapacket Doppler spread. The basic technique equalizes multipath dispersion commensurate with the bandwidth of the baseband, pre-spread, message signal. This multipath equalization operation can be extremely simple if the bandwidth of the message signal is low. If the bandwidth of the pre-spread message signal is sufficiently low, e.g., the correlation width or inverse bandwidth of the pre-spread message signal is a large multiple of the largest multipath delay in the transmission channel, this equalization operation reduces to a complex multiply operation that is automatically subsumed into the adaptive despreading operation. This is in contrast to conventional CDMA systems, which require additional equalization operations unless the correlation width of the spread signal is a large multiple of the largest multipath delay in the transmission channel.
Another advantage of the present invention is that a radio communication method is provided that is compatible with discrete multiple tone and orthogonal frequency-division multiplex-like frequency channelization techniques. This allows stationary and linear channel distortion to be modeled as an exactly multiplicative effect on the transmit spreading code.
An advantage of the present invention is that a radio communication method is provided that is compatible with time-division duplex systems. So time-division duplex communication formats can be used where the stacked-carrier spread spectrum modulation format is packetized,.e.g., if the stacked-carrier spread spectrum signal is generated using discrete multiple tone and/or orthogonal frequency-division multiplex-like based frequency channelizers and inverse channelizers. A xe2x80x9clocalxe2x80x9d estimation of the transmit channel at either end of the communication link is made possible, greatly simplifying the implementation of channel preemphasis, transmit-site channel equalization topologies and retrodirective transmission techniques.
An advantage of the present invention is that a radio communication method is provided with a code-division multiple-access type of multiple access capability, e.g., the stacked carrier multiple access technique. Point-to-multipoint communication links are implemented by transmitting signals over the same subset of frequency channels, using linearly independent, (orthogonal or non-orthogonal) sets of spreading gains to separate the signals at the despreader. Since the spreading codes can be non-orthogonal, a chief advantage of the present invention when used in conjunction with code nulling techniques, is that the use of non-orthogonal spreading codes is possible.
An advantage of the present invention is that a radio communication method is provided that is compatible with xe2x80x9cbandwidth-on-demandxe2x80x9d flexible data rate techniques. The data rate supplied on a given link can be increased or decreased in small increments by transmitting primitives to a single user over multiple time, frequency, or stacked carrier channels. The data rate is then adjusted with no increase in bandwidth if the data rate increases using multiple stacked carrier channels.
An advantage of the present invention is that a radio communication method is provided that is compatible with high-order digital modulations. It is compatible with arbitrary Mary digital baseband modulation formats and allows capacity improvement through transmission of higher numbers of bits/symbol on each frequency channel. The reuse is improved and xe2x80x9cload balancingxe2x80x9d in multicell communication networks can be included by varying the bits per symbol on each primitive.
An advantage of the present invention is that a radio communication method is provided that has space-division multiple-access, interference excision, and channel equalization capability, e.g., in the code nulling technique. Such space-division multiple-access like code nulling techniques, in optimal and quasi-optimal linear interference cancellation and signal extraction techniques, are useful to separate stacked-carrier spread spectrum signals at the despreader, based on the frequency diversity or spectral diversity of the signals. Interference excision against in-cell stacked-carrier spread spectrum signals are thereby provided, as well as the elimination of out-of-cell interferers, e.g., the reuse enhancement capability. This then allows the most effective use of code nulling, which-is generally applicable to a wide range of spreading formats. In particular, such provides a factor of two capacity improvement over code-nulling techniques developed for use with modulation-on-symbol direct-sequence spread spectrum formats where the spreading gain repeats once every underlying message symbol.
An advantage of the present invention is that a radio communication method is provided that can be used with adaptive antenna arrays.
An advantage of the present invention is that a radio communication method is provided that is compatible with advanced array adaptation techniques and thereby separates signals based on spatial diversity, frequency spectrum diversity, and combined spatial/spectral diversity.
An advantage of the present invention is that a radio communication method is provided that is compatible with retrodirective communication techniques. This enables a straightforward extension of spatial retro-directivity technique to stacked-carrier spread spectrum systems including single antennas or antenna arrays. And allows concentration of most complex operations at the base station in point-to-multipoint communication links, greatly reducing the cost of the overall system.
A still further advantage of the present invention is that a radio communication method is provided that is backward compatible with conventional code-division multiple access, data activation techniques.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.